PROJECT SUMMARY/ABSTRACT Recent sequencing data has revealed an unanticipated level of genomic instability in cancer. The origin of these changes is largely unknown. This proposal focuses on two aspects of the maintenance of genome integrity relevant to cancer: telomere dysfunction and DNA repair. We propose to examine the molecular basis of the telomere tumor suppressor pathway whereby telomere attrition leads to dysfunctional telomeres and proliferative arrest. These experiments will make use of normal human epithelial cells in which critical genes have been removed with CRISPR/Cas9 genome editing and will apply innovative imaging of telomere structure using super-resolution STORM imaging in combination with expansion microscopy. When the telomere tumor suppressor pathway fails because of the loss of p53 and Rb, telomeres continue to shorten during early tumor development, leading to large numbers of dysfunctional telomeres and genome instability (referred to as telomere crisis). We propose to determine the changes in the genome that result from progression through telomere crisis. These experiments will involve a new in vitro model for telomere crisis in epithelial cells combined with whole genome sequencing. Finally, we propose to study the mechanism of double-strand break (DSB) repair, which is carefully regulated to maintain genome integrity. In particular, we will focus on the mechanism by which 53BP1 represses the resection of DSBs and promotes their mobility using the mouse model systems we have developed and new technologies for the analysis of chromatin structure (ATAC sequencing) and locus position (DamID-LaminB1 marking). These two attributes of 53BP1 are highly relevant to the induction of lethal genome instability resulting from inhibition of the PARP1 poly(ADP-ribose) polymerase in BRCA-deficient cells. Collectively, the proposed experiments will illuminate new aspects of genome instability in cancer. The objective of these studies is to gain insights that will ultimately inform and guide decisions in the cancer clinic.